![Thor-Able III Explorer 6](https://upload.wikimedia.org/wikipedia/commons/8/83/Thor-Able_III_Explorer_6.jpg)
Explorer 6, or S-2, was a
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
satellite, launched on 7 August 1959, at 14:24:20
GMT
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...
. It was a small, spheroidal satellite designed to study trapped radiation of various energies, galactic
cosmic ray
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s,
geomagnetism,
radio propagation
Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere.
As a form of electromagnetic radiation, like light waves, radio waves are affecte ...
in the
upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes:
* The mesosphere, which on Earth lies between the ...
, and the
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
of
micrometeorites
A micrometeorite is a micrometeoroid that has survived atmospheric entry, entry through the atmosphere of Earth, Earth's atmosphere. Usually found on Earth's surface, micrometeorites differ from meteorites in that they are smaller in size, more a ...
. It also tested a scanning device designed for photographing the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's
cloud cover
Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud co ...
.
[ ] On 14 August 1959, Explorer 6 took the
first photos of Earth from a satellite.
Experiments
Beacon (108 and 378 MHz)
This experiment measured the
electron density
In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...
near the satellite. The observational equipment comprised two coherent transmitters operating at 108 and 378
MHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
. Doppler difference frequency and change in
Faraday
Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, ...
rotation of the 108-MHz signal were observed. Signals were observed from the receiving station at
Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
for 20 to 70 minutes during each of eight passes during 11 days. Severe fading and a strong magnetic storm added to difficulties in data interpretation. The 378-MHz beacon transmitter failure terminated the experiment.
[ ]
Fluxgate Magnetometer
A
fluxgate magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, on ...
was used to measure the component of the
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
parallel to the spin axis of the vehicle. The measurements, when combined with those made with the search coil magnetometer (which measured component of the ambient field perpendicular to spin axis of vehicle) and the aspect sensor, where intended to determine the direction and magnitude of the ambient magnetic field. It was intended to obtain measurements at altitudes up to 8 Earth radii, but due to permanent multipole disturbances within the vehicle the fluxgate magnetometer became saturated and returned no data. Thus, information was available from only the search coil and the aspect indicator.
[ ]
Ion Chamber and Geiger–Müller Counter
The instrumentation for this experiment consisted of a Neher-type integrating
ionization chamber
The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...
and an Anton 302
Geiger–Müller tube
The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation. It is named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated w ...
(GM). Due to the complex nonuniform shielding of the detectors, only approximate energy threshold values were available. The ion chamber responded omnidirectionally to
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s and
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s with energies greater than 1.5 and 23.6
MeV
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an Voltage, electric potential difference of one volt i ...
, respectively. The GM tube responded omnidirectionally to electrons and protons with energies greater than 2.9 and 36.4 MeV respectively. Counts from the GM tube and pulses from the ion chamber were accumulated in separate registers and telemetered by the analog system. The time that elapsed between the first two ion chamber pulses following a data transmission and the accumulation time for 1024 GM tube counts were telemetered digitally. Very little digital data were actually telemetered. The ion chamber operated normally from launch through 25 August 1959. The GM tube operated normally from launch through 6 October 1959.
[ ]
Micrometeorite
A
micrometeorite
A micrometeorite is a micrometeoroid that has survived entry through the Earth's atmosphere. Usually found on Earth's surface, micrometeorites differ from meteorites in that they are smaller in size, more abundant, and different in composition. T ...
detector (micrometeorite momentum spectrometer), which employed piezoelectric crystal microphones as sensing elements, was used to obtain statistics on the momentum flux and the variations of flux of micrometeorites. Although pulses were detected, the experiment returned no data of scientific value.
[ ]
Proportional Counter Telescope
A triple-coincidence omnidirectional proportional counter telescope was used to observe protons (with E>75
MeV
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an Voltage, electric potential difference of one volt i ...
) and electrons (with E>13 MeV) in the terrestrial trapped radiation region. The scientific objective of the telescopes was to determine some of the properties of high-energy radiation in
interplanetary space
Interplanetary may refer to:
*Interplanetary space, the space between the planets of the Solar System
*Interplanetary spaceflight, travel between planets
*The interplanetary medium, the material that exists in interplanetary space
*The InterPlanet ...
, including the proportion of counts due to
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
s versus those due to protons and other
high-energy particles. Comparison with results from the Cosmic Ray Ionization Chamber makes it possible to determine the type and energy of particles responsible for the measurement.
Each telescope consists of seven proportional counter tubes, six in a concentric ring around the seventh running parallel along their lengths. These bundles of tubes lie on their sides projecting through the top of one of the equipment boxes in the hexagonal base of
Ranger 1
Ranger 1 was a prototype spacecraft launched as part of the Ranger program of unmanned space missions. Its primary mission was to test the performance of those functions and parts necessary for carrying out subsequent lunar and planetary mission ...
. Three of the outer tubes are exposed to space and three project into the equipment box. Each set of three is connected electronically into a group that feeds into a pulse amplifier and pulse shaper. The central tube feeds into its own equivalent circuit.
The two telescopes were designated a "low-energy" and "high-energy" telescope, differing only in the amount of shielding and its configuration. The counters in the high-energy telescope were 3-inch long, 0.5 inch diameter
brass
Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other with ...
tubes with a thickness of 0.028 inches. A
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
shield of 5 grams per cm
2 thickness surrounds the entire assembly. The low-energy unit has the same size tubes, but made of
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
with a wall thickness of 0.508 ± 0.0025-mm. Half the assembly has 5 grams per cm
2 lead shielding along the length of the tubes. The unshielded half of the assembly is the exposed portion that particles can reach without encountering spacecraft structural material, giving an angular resolution of under 180° for low-energy particles. The low-energy telescope can detect protons with energies greater than or equal to 10 MeV and electrons greater than or equal to 0.5 MeV. The high-energy telescope detects 75 MeV and above protons and 13 MeV and above electrons in triple-coincidence, and bremsstrahlung above 200 keV in the central tube.
When a particle passes through the bundle of tubes, the electronic circuit determines which groups have been penetrated. If a pulse comes from all three groups at once, a triple-coincidence, the particle was a high-energy one, rather than a low-energy one or an X-ray. The triple-coincidence events are telemetered together with the single counts from the center tube to determine counts due to high-energy versus low-energy sources. The high-energy telescope counting rate allows correction of the low-energy telescope data in order that the particle flux incident on the unshielded portion of the low-energy unit can be calculated. Comparing data from the low-energy telescope and the Cosmic-Ray Ionization Chamber (both detect particles in the same energy range) makes it possible to determine the average ionization per particle, from which the type and energy of the particle can be determined.
Several magnetic storms occurred during the active life of the experiment. The date of transmission of the last useful information was on 6 October 1959, after which the transmitter failed to operate.
[ ]
Scintillation Counter
The scintillation counter experiment was designed to make direct observations of electrons in the Earth's radiation belts with a detector insensitive to
bremsstrahlung
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
. This experiment consisted of a cylindrical plastic scintillator cemented to a photomultiplier tube. The instrument viewed space through a foil-covered window in the payload shell, but the instrument also responded to more energetic particles passing through the payload shell. The minimum energies detectable were 200 keV for electrons and 2 MeV for protons. For electrons between 200 and 500 keV, the detector efficiency times the omnidirectional geometric factor was 0.0008-cm
2 count per electron; whereas for electrons of energy greater than 500 keV, it was 0.16-cm
2 count per electron. For very penetrating particles, the geometrical factor rose to its maximum value of 3.5-cm
2. The scintillation counter was sampled continuously for analog transmission and intermittently (every 2 minutes, 15 seconds, or 1.9 seconds, depending upon the satellite bit rate) for digital transmission. The transmitter broadcasting the analog data for this experiment failed on 11 September 1959. Data were received on a limited duty cycle from the digital transmitter until early October 1959.
[ ]
Search-Coil Magnetometer
This experiment was designed to survey the gross magnetic field of the Earth, to investigate the
interplanetary magnetic field
The interplanetary magnetic field (IMF), now more commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar Syst ...
, and to detect evidence of any lunar magnetic field. No interplanetary or lunar magnetic fields could be measured, however, because of the spacecraft's low apogee. The instrument was similar to that flown on
Pioneer 1
Pioneer 1 (also known as Able 2) was an American space probe, the first under the auspices of NASA, which was launched by a Thor-Able rocket on 11 October 1958. It was intended to orbit the Moon and make scientific measurements, but due to a g ...
and consisted of a single search coil mounted so that it measured the magnetic field perpendicular to the spacecraft spin axis. The instrument had a range of 0.6 nT to 1200 nT. No inflight calibration was provided. Some degradation of the telemetry signal occurred due to ionospheric effects. Insufficient ground observations on the electron content of the ionosphere prevented correcting the data for these effects. The experiment had both digital and analog outputs. The magnetometer amplitude and phase were sampled continuously for analog transmission and intermittently (every 2 minutes, 15 seconds, or 1.9 seconds, depending on satellite bit rate) for digital transmission. The magnetometer worked until loss of the telemetry signal in early October 1959.
[ ]
TV Optical Scanner
The TV optical scanner flown was an improved version of the TV system first employed on
Pioneer 2
Pioneer 2 (also known as Able 3) was the last of the three project Able space probes designed to probe lunar and cislunar space. The launch took place at 07:30:21 GMT on 8 November 1958. After Pioneer 1 had failed due to guidance system deficien ...
. The experiment consisted of an optical unit containing a concave spherical mirror and phototransistor, a video amplifier, timing and logic circuits, and telemetry. The experiment was designed to test the feasibility of using such instrumentation to obtain low-resolution daylight cloudcover photographs. The scanner also served as a forerunner to the TV camera systems carried on later, more advanced satellites. The scanner's optical axis was directed 45° away from the spacecraft spin axis, which was parallel to the orbital plane. The vehicle's spin furnished the line scanning, and the spacecraft's forward motion along its trajectory provided the frame scanning. During a scan (one spacecraft revolution), a single scan spot (element) on Earth was viewed and transmitted back to Earth. During the next spacecraft revolution, an adjacent spot was scanned. This procedure was repeated until a line of 64 such spots was formed. Then the process was repeated to form an adjacent line of elements, and so on, until a frame, or picture, was obtained. The system could produce useful photographs only when the spacecraft's velocity and orbital position were such that successive lines overlapped. (At apogee, for example, the TV lines were separated by a distance about equal to their length, and hence no meaningful picture could be obtained). Data obtained from this experiment are limited and of extremely poor quality. Proper spacecraft orientation was never achieved, resulting in a considerable amount of blank space between successive scan lines. The scanner's logic circuits also failed to operate normally (only every fourth scan spot could be successfully reproduced), further reducing the resolution. The last useful data were obtained on 25 August 1959.
[ ]
VLF Receiver (15.5 kHz)
This
Very low frequency
Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave a ...
(VLF) receiver was designed to study Whistler mode propagation and ionospheric moise on 15.5 kHz signals transmitted from
Annapolis, Maryland
Annapolis ( ) is the capital city of the U.S. state of Maryland and the county seat of, and only incorporated city in, Anne Arundel County. Situated on the Chesapeake Bay at the mouth of the Severn River, south of Baltimore and about east o ...
. The signals were received on a small electric antenna which was simultaneously used to transit
Very high frequency
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves ( radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
(VHF) telemetry. The signal intensity on a 3-
db bandwidth of 100 hz was observed along with the antenna impedance. The dynamic range of the receiver was about 80 db. This experiment operated from launch up to about 160 km before failure. With the antenna in a folded configuration for launch, the receiver recorded all data at a sensitivity reduced by about 30 db. At , the signals disappeared into the noise background. However, by special techniques, data were made usable all the way up to .
[ ]
Launch
The satellite was launched into a
highly elliptical orbit
A highly elliptical orbit (HEO) is an elliptic orbit with high eccentricity, usually referring to one around Earth.
Examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used them, ...
, on 7 August 1959, at 14:24:20 GMT.
First image of Earth by a satellite
![First satellite photo - Explorer VI](https://upload.wikimedia.org/wikipedia/commons/f/fd/First_satellite_photo_-_Explorer_VI.jpg)
On 14 August 1959, Explorer 6 took the first image of Earth ever by a satellite. It was over
Mexico
Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
at an altitude of approximately . The image was a very crude picture of the north central
Pacific Ocean
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
, transmitted to a ground station in
Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
over a 40 minute span.
ASAT test
On 13 October 1959, an
anti-satellite missile
Anti-satellite weapons (ASAT) are space weapons designed to incapacitate or destroy satellites for strategic or tactical
purposes. Several nations possess operational ASAT systems. Although no ASAT system has been utilised in warfare, a few ...
(ASAT) test of the
Bold Orion
The Bold Orion missile, also known as Weapons System 199B (WS-199B), was a prototype air-launched ballistic missile (ALBM) developed by Martin Aircraft during the 1950s. Developed in both one- and two-stage designs, the missile was moderately suc ...
missile used Explorer 6 as a target. The missile successfully passed within of the satellite. Launch took place within the
Atlantic Missile Range
The Eastern Range (ER) is an American rocket range (Spaceport) that supports missile and rocket launches from the two major launch heads located at Cape Canaveral Space Force Station and the Kennedy Space Center (KSC), Florida. The range ha ...
Drop Zone
A drop zone (DZ) is a place where parachutists or parachuted supplies land. It can be an area targeted for landing by paratroopers, or a base from which recreational parachutists and skydivers take off in aircraft and land under parachutes. In ...
(AMR DZ). The altitude, latitude and longitude of the drop point were , 29° North and 79° West, respectively. Bold Orion successfully intercepted the Explorer 6 satellite, passing its target at a range of less than and an altitude of .
Mission
The satellite was
spin-stabilized at 2.8 rotation per seconds (rps), with the direction of the
spin axis
Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rot ...
having a
right ascension
Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the earth.
When paired w ...
of 217° and a
declination
In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
of 23°. Four
solar cell
A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. paddles mounted near its equator recharged the storage batteries while in orbit. Each experiment except the television scanner had two outputs, digital and analog. An
Ultra high frequency
Ultra high frequency (UHF) is the ITU designation for radio frequency, radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one ten ...
(UHF) transmitter was used for the digital telemetry and the TV signal. Two
Very high frequency
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves ( radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
(VHF) transmitters were used to transmit the analog signal. The VHF transmitters were operated continuously. The UHF transmitter was operated for only a few hours each day. Only three of the solar cell paddles fully erected, and this occurred during spin up rather than prior to spin up as planned. Consequently, initial operation of the payload power supply was 63% nominal, and this decreased with time. The decreased power caused a lower signal-to-noise ratio affecting most of the data, especially near apogee. One VHF transmitter failed on 11 September 1959, and the last contact with the payload was made on 6 October 1959, at which time the solar cell charging current had fallen below that required to maintain the satellite equipment.
The satellite's orbit
decayed on 1 July 1961.
A total of 827 hours of analog and 23 hours of digital data were obtained.
See also
*
First images of Earth from space
This is a timeline of first images of Earth from space. The initial photographs and digital images of planet Earth taken from outer space were preceded by aerial photography and continue in the form of satellite imagery.
For the purpose of this ...
*
Operation Argus
Operation Argus was a series of United States low-yield, high-altitude nuclear weapons tests and missile tests secretly conducted from 27 August to 9 September 1958 over the South Atlantic Ocean. The tests were performed by the Defense Nucle ...
*
Explorer program
The Explorers program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United Stat ...
References
External links
NASA's Explorer MissionsSpace Technology Laboratories Documents ArchiveScientific findings from Explorer VI
{{Orbital launches in 1959
Spacecraft launched in 1959
Explorers Program
Spacecraft which reentered in 1961
Earth observation satellites